The current proposal is a continuation of a grant that the investigator received in 2001. The continuing hypothesis of this project is that olfactory axons from the nose are necessary for normal maintenance of the post-synaptic organization of the olfactory bulb in adult zebrafish. The PI has shown that permanent peripheral deafferentation, by unilateral olfactory-organ ablation, has a significant effect on the adult olfactory bulb in this animal, including a decrease in bulb volume and total cell number, an increase in cell death, and changes in protein expression. The goal of the current proposal is to begin to address the mechanisms responsible for deafferentation induced changes in the adult olfactory bulb in this animal. The effects seen after removal of the primary afferent axons may be due to loss of sensory input (activity-dependent mechanisms) or loss of structural contact (contact-dependent factors). It is possible that activity in the bulb is crucial for bulb maintenance: ablation of the olfactory organ removes the primary source of activity. Another possibility is that contact between the olfactory axons and the bulb is responsible for continual support of bulbar neurons: removal of the axons eliminates this contact. The focus of this proposal is to separate these two hypotheses by completing the following three specific aims. Specific Aim 1: Does complete activity blockade cause changes in the olfactory bulb that are similar to olfactory-organ ablation? Specific Aim 2: Does removal of only odor-induced activity cause the same changes in the olfactory bulb as physical deafferentation? Specific Aim 3: Does transection of the olfactory nerve induce the same effect as ablation of the olfactory organ?